1. Field of the Invention
This invention relates to semiconductor devices, and in particular to a method of simultaneously forming epitaxial resistors, base resistors, and vertical transistor bases in integrated circuit structures.
2. Prior Art
Oxide isolated integrated circuit structures are well known and have been employed in integrated circuit technology for a substantial period. See e.g., U.S. Pat. No. 3,648,125 entitled "Method of Fabricating Integrated Circuits with Oxidized Isolation and the Resulting Structure" issued March 7, 1972, to Douglas Peltzer, and referred to hereinafter as the Peltzer patent. Equally well known are techniques for fabricating epitaxial resistors, base resistors, and vertical transistor bases. See also the Peltzer patent.
Rather than utilize ion implantation techniques of fabrication, the prior art relied primarily upon diffusion processes to produce the desired semiconductor structures. Accordingly, difficulty was often encountered in precisely positioning the desired quantity and concentration of impurities within a given semiconductor structure. For example, in the fabrication of base resistors it is desirable to have a very high concentration of impurity at the surface of the semiconductor structure so excellent ohmic contact may be made to the base resistor. It is also desirable, however, when fabricating high resistivity base resistors, to have low impurity concentrations within the body of the base resistor, as it is well known that resistivity is inversely proportional to impurity concentration. That is, the greater the amount of impurity, the lesser the resistivity. These structures, as formed by prior art processing often have undesirably high concentrations of impurities below the semiconductor substrate surface.
For the same reasons as discussed above in conjunction with base resistors, it is also desirable to have very low impurity concentrations in the body of epitaxial resistors, but high concentrations in the ohmic contact regions. With the processes utilized in forming prior art structures it was difficult to obtain the desired concentration profiles. Relatively large amounts of impurity were required to obtain the desired impurity concentration at the surface, with the unattractive secondary result that an undesirable amount of impurity would diffuse into the regions of the structure away from the surface. This excess concentration of impurity within the body of the structure lowered the resistivity.
Prior art technology also relied upon diffusion processes to position impurity concentrations in the active bases of vertical transistors. The present invention provides a technique whereby most of the impurity concentration is placed in the active region below the emitter-base junction. This provides a higher level of beta control than prior art thermal processing.
One frequent problem with many prior art integrated circuit structures was their susceptibility to inversion phenomena. Inversion phenomena are well known in the art of semiconductor manufacture and involve the changing of the surface of semiconductor material from one conductivity type to the opposite type. For example, a low concentration of P type semiconductor material can be inverted to N type material by the inadvertant inclusion of relatively few sodium ions in the oxide covering the material. The present invention avoids this difficulty of prior art devices by providing a high surface doping concentration, but a low total bulk concentration.
Finally, prior art semiconductor fabrication processes required numerous steps to produce a structure having epitaxial resistors, base resistors, and vertical transistor bases. For example, in the Peltzer patent, itself a substantial advance over previous art, several steps were required to define the base contact predeposition region, the regions in which epitaxial resistors would be formed, and the regions in which base resistors would be formed. The present invention circumvents this cumbersome procedure by defining all the above regions with only two masks, and then forming epitaxial resistors, base resistors, and vertical transistor bases simultaneously with a single ion implantation.